Measurement of urea nitrogen and creatinine concentrations in peritoneal dialysate and other body fluids using the Vitros analyzer

Objectives: A majority of chemistry analyzers have separate modes for estimating urea nitrogen and creatinine in serum and urine because of significant differences in their concentration. As there is no defined mode for other biological fluids such as dialysis fluid, these can be run on either of the two modes. On Vitros (Clinical Ortho-Diagnostics, Rochester, NY) chemistry analyzer, we observed an overestimation in calculated creatinine clearance (CrCl) and Kt/Vurea in some children that led us to investigate into methodology used for these assays. Design and methods: We measured urea nitrogen and creatinine on peritoneal dialysate effluent, pleural fluid and hemofiltration effluent on serum and urine modes of Vitros analyzer. Results: Vitros chemistry analyzer significantly overestimated urea nitrogen and creatinine in ‘urine’ mode on samples with low urea nitrogen and creatinine. Conclusions: Since peritoneal dialysate effluent has concentrations of urea nitrogen and creatinine comparable to serum; hence, it should be analyzed on ‘serum’ mode.